As portable electronic devices become smaller, there is a corresponding decrease in the area for external device connectors commonly used for accessories. There are also practical limits to external connector density and size, because such connectors are intended to be as durable and reliable as possible. Not only is the goal of a smaller connector in conflict with durability and reliability goals, but it also imposes limits on the number of connector pins. Fewer connector pins limits functionality and/or reduces flexibility.
Today's two-way radios connect to an array of accessories that are of varying complexity. Some of these accessories contain a microprocessor and are considered “smart”, while others are less complex (hereafter, “simple accessories”) and intended to be low cost. Smart accessories typically communicate with the radio through a data bus, while simple accessories tend to use a number of discrete digital lines and have no microcontroller. Smart accessories may also require discrete digital lines in addition to the data bus for special functions. Accessories may often have additional lines for audio, which further increases pin count. Although sophisticated digital interfaces are available that can handle audio, data, and control signals, the cost and complexity of such an interface is often prohibitive. A desired accessory interface must be flexible enough to meet the needs of both smart and simple accessories without requiring excessive pin count, excessive cost, or excessive complexity.
Additionally, it is important to have a common accessory interface for all radio accessories, including accessories not yet developed. It is strongly desired to minimize changes to the radio to support such accessories. Although some radios are intended to be flash-upgradeable, many lower-cost radios are not upgradeable. It is important that lower-cost radios be able to utilize as many future accessories as possible. It is also desirable for the least complicated or most simple accessories to work on the widest possible range of radios.
Accordingly, there is a need for a common accessory interface that meets the needs of both smart accessories and simple accessories, while providing maximum flexibility for configuring the interface. There is also a need to keep the pin count of the accessory interface as small as possible to minimize cost and complexity.